Incoming VPN Connections Fail on AirPort Extreme
Main detailed post over on SuperUser: http://cl.ly/0X3c2b0F3q2K1T1X2F1i
Summary: I have an AirPort Extreme hooked up to a cable modem; an iMac server is wirelessly connected to that and has a static internal IP. It has VPN (the one built-in to Mac OS X) and SSH set up, and the Extreme has port forwarding on TCP 22 for SSH and UDP 500/1701/4500 for VPN, and also a few extras for ARD. SSH connects fine (my dynamic DNS host works fine, and that port is forwarding without trouble) but VPN fails. I had the exact same ports forwarded before using a different router and it worked fine, and VPN also works fine within the local network.
I called Apple and they told me that they couldn't help me; they said that "as long as you can get on the internet then the router is working fine". Clearly there's a problem but nothing I've tried has solved it.